Not sure exactly what you're trying to do, but this will get you started. Put your shockwave layer above your footage layer. Set your footage layer's track matte to "Alpha Matte Inverted". When the shockwave appears, the footage underneath will begin to disappear.

Basically im am trying to get footage of someone's head to be masked away by the shockwaves revealling just a background behind them. Its like the shockwave is eating away the body and making them disappear.

So at first its just the actors head but then the shockwave starts and consuming the body and making the actor disappear.